1. Field of the Invention
The present invention relates generally to calibrating electrical circuits, such as signal filters. In particular, the present invention relates to an electrical calibration circuit.
2. Background and Related Art
Electrical circuits are often fabricated by processes that introduce unexpected or unwanted characteristics. Such characteristics may vary a circuit's performance from its designed specifications. External factors such as temperature can also vary the performance of a circuit.
A Sallen and Key (“SK”) low pass filter is depicted schematically in FIG. 1. As will be appreciated by those skilled in the art, the illustrated SK filter provides a second-order response. As shown in FIG. 1, the SK filter includes a known 1× buffer 1, resistors R1 and R2, and capacitors C1 and C2. A straightforward analysis of the SK filter yields the following transfer function, with R2=R1:                                           H            ⁡                          (              s              )                                =                      1                                          S                2                            +                                                Wp                  Q                                ⁢                S                            +                              Wp                2                                                    ,                            (        1        )            where                               Wp          =                      1                                                            R                  1                                ⁢                                  R                  1                                ⁢                                  C                  1                                ⁢                                  C                  2                                                                    ,                                   ⁢        and                            (        2        )                                          1          Q                =                              2            ⁢                                                                                R                    1                                    ⁢                                      C                    2                                                                                        R                    1                                    ⁢                                      C                    1                                                                                =                      2            ⁢                                                                                C                    2                                                        C                    1                                                              .                                                          (        3        )            
SK Filters are often employed in circuits that require precise signal responses. However, such filters are rendered ineffective if their characteristics, such as a cutoff frequency (ωc), are altered in the fabrication process. For example, a design may call for a 10 pF capacitor, but the fabricated capacitor may actually have a capacitance of 11.5 pF. Process variance is particularly problematic for integrated circuits, since integrated components are not easily calibrated.
Conventional calibration techniques, such as calibrating for a precise resistance or capacitance, are insufficient because both resistance and capacitance can vary, and the cutoff frequency (ωc) is a function of the RC product.
Hence, there are many accuracy-related problems to be solved in the art. Accordingly, an electrical calibration circuit is needed in such circuits. There is a further need for an electrical calibration circuit to calibrate signal filters. There is also a need for a calibration method to correct for fabrication variance.